AOA,
you can access servers by IP which has assigned on modem. I don't know what network and server settings you have. but I can give you idea.
1) RDP port for every server is 3389, if u want to change its your desire.
2) if you change IP need to make rule in windows firewall for incoming RDP connection with new port for example 3359 or 33890
3) you need to make some rules for "ip nat inside source static" in modem.
e.g;
# ip nat inside source static tcp 192.168.1.2 3389 interface Dialer0 3389
or
#ip nat inside source static tcp 192.168.1.2 33891 interface Dialer0 33891
#ip nat inside source static tcp 192.168.1.3 25 interface Dialer0 25
#ip nat inside source static tcp 192.168.1.101 443 interface Dialer0 443
I hope you will get idea. pls let me know if you need more help.
thanks
Khan